I have the Post Indexer plugin installed in several WPMU installs and I have noticed that sometimes new post doesn't get indexed even if all indexing settings are configured correctly.
I explored the issue and found that save post hook doesn't get applied every time I publish a new post.
I solved the issue by adding two extra hooks to classes/class.postindexeradmin.php
add_action('edit_post', array(&$this, 'index_post'), 99, 2 );
add_action('publish_post', array(&$this,'index_post'), 99, 2 );
After adding these two hooks everything seems to be working correctly.
I would be glad if this fix can be included to the next release. I could be hard to remember to fix the problem after every update.